The present invention is generally related to noise reduction techniques and devices for pneumatically driven turbine machinery and, more particularly, to noise reduction techniques and devices for pneumatically driven hydraulic pumps of the type utilized to power various systems within an aircraft. Before turning to the present invention, the following brief description of the pertinent operating environment is set forth as a background to the noise problem to which the invention is directed.
Pneumatically driven hydraulic pumps are commonly employed in commercial transport airplanes to provide the hydraulic power required by various hydraulic actuators and motors such as those that drive the landing gear, flaps, and spoilers. Such pump assemblies generally include a pneumatically driven turbine which drives a hydraulic pump unit, with the entire assembly contained as a unit in a housing or casing. The turbine is typically driven with bleed air supplied by one or more of the gas turbine engines that propel the aircraft. The pressurized hydraulic fluid provided by the pump is metered to the various hydraulic actuators in a conventional manner.
In one particular type of pneumatically driven pump assembly employed on large commercial airplanes, the shaft of the air turbine is coupled through a reduction gear assembly to the main shaft of a variable-displacement rotary pump. Engine bleed air is supplied to the turbine through a modulating valve which meters the airflow in response to hydraulic demand and thereby controls the turbine speed as well as the speed of the pump unit. The pump assembly additionally includes a solenoid-actuated shutoff valve in the air supply duct, which operates in response to a signal from a turbine overspeed sensor, to interrupt pneumatic power to the turbine in the event of an overspeed condition.
The pump assembly described above is ordinarily located in the fuselage of the airplane. Like other air turbine machinery, the pump assembly emits substantial levels of noise. Because of its proximity to the passenger compartment in the case of a commercial airliner, the pump has been an object of noise reduction efforts. In this regard, it has been recognized that a significant proportion of the acoustic energy generated within the pump assembly is emitted from the turbine scroll of the pump assembly casing and is transmitted inwardly through the surrounding enclosure structure to contribute to cabin noise in the passenger compartment. Another major portion of the acoustic energy is transmitted as exhaust noise outside the airplane, primarily through the turbine air exhaust duct which opens outwardly from the fuselage. This exhaust noise contributes to what is known as ramp noise in the ground service areas around the airplane and often reaches unacceptably high levels. Accordingly, as part of an overall effort to make commercial airplanes quieter, and to reduce cabin noise and ramp noise in particular, it has been sought to reduce the noise level of this type of pump assembly.
The noise emitted by the pump assembly is somewhat characteristic of turbine noise in frequency and spectral distribution. Since pneumatically driven turbines are well known to emit high levels of noise, it has been largely assumed previously that the turbine within the pump assembly is the major source of noise. Since it is generally accepted that there is no practical way to greatly reduce the generation of noise by the turbine without impairing its efficiency, previous efforts toward noise reduction have been directed to containing or absorbing the noise emitted by the pump assembly, rather than reducing the generation of the noise at its source. In this regard, as noted above, it has been recognized previously that most of the noise is radiated in two modes--as case-radiated noise and as exhaust-radiated noise. Exhaust-radiated noise has been attenuated, for example, by providing a muffler in the air exhaust duct leading from the turbine. Case-radiated noise has been attenuated by enclosing or surrounding the entire assembly with an acoustically absorbent material. However, these approaches have not been altogether satisfactory because they add weight to the pump assembly and hinder access for servicing and maintenance.
Accordingly, it is the primary object and purpose of the present invention to provide a method and apparatus for reducing the level of noise generated by pneumatically driven turbine machinery, particularly a pneumatically driven hydraulic pump in an airplane. More specifically, it is an object of the invention, for reasons that will become apparent from the following discussion, to provide a method and apparatus for reducing noise emission from turbine machinery wherein a modulating valve regulates flow of gas from a source of high pressure gas to a turbine.
It is a further object of the present invention to achieve the foregoing objects in a pneumatically driven hydraulic pump in an airplane without significantly increasing the weight of the assembly.
It is another object, also for reasons that will become apparent from the following discussion, to provide a method and apparatus for reducing the level of noise generated by a pneumatically driven hydraulic pump that is controlled by a modulating valve, which method and apparatus does not diminish the control responsiveness of the turbine with respect to the modulating valve.